I've never used mod_deflate so cant really offer much assistance, this is really a question for an Apache forum.
most examples I am seeing of the FilesMatch code are using 2 slashes:
<FilesMatch "\\.(js|css|html|htm|php|xml)$">
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.